To determine the proportion and features of invasive interval cancers that could be detected at the time of screening and the proportion classified as true-intervals, false-negatives, minimal signs or radiographically occult lesions.

BreastScreen Victoria (Australia).

Review methodologies adopted: (1) a blinded review by five readers of interval, screen-detected and normal cases, followed by a confirmation exercise to determine the proportion detectable at the time of the previous screen; (2) an unblinded review to classify interval cases as true-interval, false-negative, minimal signs or radiographically occult.

From the blinded review, 38% of interval cases were considered `potentially detectable' at screening. Comparison of interval and screen-detected cases shows that interval cases are more likely to be smaller, equivocal and ill-defined masses. In the unblinded exercise, 41% of interval cases were classified as false-negatives, 16% as minimal signs, 33% as true-intervals and 10% as radiographically occult.

This highlights the importance of adopting staged review methods with blinded and unblinded components. The blinded review and confirmation exercise determines the proportion of interval cases that were detectable at screening. The unblinded review provides an opportunity for professional development and links into the blinded review through further classification of interval cases.